During produce season, I could do it easily. VERY easily. And, I could eat very well a couple times a week. I'd do all my shopping for the month at a farmer market auction. Bushels of sweet corn go for as little as $3 or $4. I can pick up 100 pound bags of potatoes for $7 or $8. They practically give away boxes of things like squash. Plus, a lot of farmers market types of businesses take their stuff to auction before it gets too old to sell. Bananas - 50 cents for 10 pounds. Sometimes I get lucky & can get live chickens (roosters, actually) and rabbits for about 50 cents each.
So, 5 each chickens & rabbits = $5
100 pounds of potatoes = $7
Big paper grocery bag filled with corn on the cob = $2
(I'd par boil most of it & cut it off the cob so it doesn't go bad before 30 days.)
Big bag of zucchini = $1
Box of cantelopes = $2
Box of carrots = $2
Box of bananas (refrigerate & the skins turn browner, but the insides slow down in ripening.)
Box of day old bread from the bakery $2 (Strohmans a few miles from me.)
6 dozen eggs = $2
Big box of large tomatoes that are ripe and have cracks in them (cook down to make pizza sauce and spaghetti sauce - both good on bread, also cook with zucchini) $2
That leaves $5 for miscellaneous stuff like cheap 3 for $1 spices, or maybe splurge on something like butter, and I'll have food left over.
Breakfast: a couple of eggs (over easy) on dry toast, and alternate between bananas and cantelope from day to day. Lunch: baked potato, maybe something like grilled zucchini, and a banana. Dinner: meat, baked/mashed/sliced and grilled potato, corn or carrots.
edit: this ignores that I do have the space to just grow almost all of my own food, but I believe that breaks the spirit of "30 days."